The Internet of Things (IoT) is an umbrella term that covers many groups of related concepts, but in essence, these concepts share the following features: distributed intelligence, multiple interconnected sensors/actuators and decentralized control. In practice, IoT means that certain spaces, environments or objects can be made “smart” by incorporating sensors that can communicate to make them behave intelligently. All of this new technology needs power: highly efficient, low standby consumption AC/DC power supplies that can be used in a smart home or office with tens or hundreds of intelligent sensor nodes. In many cases, where there is no user access to the electrical parts is expected, these power supplies do not require safety isolation. In such cases, a sensor node can benefit from a low-cost non-isolated power supply solution.
A non-isolated flyback converter may be used to generate a low-power, low-voltage power supply. A separate auxiliary winding is used to generate a bias voltage powering the control circuitry of the converter itself. However, the cost of a multiple-winding flyback transformer puts it at a disadvantage compared to a single-inductor buck converter. A number of simple self-biased voltage regulators have been developed that use the buck converter topology and are capable of generating low output voltage from the universal AC line input.
FIG. 1 depicts a prior-art self-biased non-isolated buck regulator receiving input supply voltage VIN from an input source 101 and converting it to a low output voltage VO at a load 106. The regulator includes: a high-side high-voltage switch 102 receiving a gate drive signal G1, a high-voltage rectifier diode 103, an inductor 104, an output filter capacitor 105. The regulator additionally includes a bootstrap diode 107 and a flying capacitor 108 to derive bias voltage VBOOT for driving the switch 102.
The regulator of FIG. 1 suffers certain disadvantages. A high-voltage (˜600V) ultra-fast diode is needed for the bootstrap diode 107, which affects the cost of the regulator. The minimum achievable output voltage is dictated by the gate voltage requirements of the switch 102. Therefore, the output voltage of 3.3V or 5V often required by the application may not be achievable with the regulator of FIG. 1. Yet another limitation is the short conduction time of the switch 102 dictated by the high step-down ratio VO/VIN, hence, the peak current in the switch 102 is difficult to control. The latter limitation affects the maximum achievable switching frequency of the regulator and, therefore, the size and cost of the inductor 104.
A self-biased prior-art regulator not having the above restriction for the output voltage and requiring no high-voltage bootstrap diode is shown in FIG. 2. The regulator additionally includes a low-voltage switch 109 receiving the gate drive signal G1. The high-voltage bootstrap diode 107 of FIG. 1 has been replaced with a low-voltage bootstrap diode 110.
FIG. 3 depicts voltage and current waveforms illustrating operation of the regulator of FIG. 2. The current IL in the inductor 104 is given by the waveform 201. The waveform 200 shows the current in the bootstrap diode 110. The gate drive signals G1 and G2 are represented by the waveforms 202 and 203, respectively. To supply the bootstrap voltage VBOOT at the capacitor 108, the trailing edge of G2 is delayed with respect to G1, maintaining the switch 102 in conduction while the switch 109 is off. Thus, the current IL finds its way through the diode 110, charging the capacitor 108 to a desired voltage level VBOOT. Under this operating mode, the gate voltage of the switch 102 is exposed to a voltage level of approximately 2·VBOOT, which negatively affects the cost and the complexity of the control circuitry generating G2. The regulator of FIG. 2 still suffers the limitations related to the short conduction time of the switches 102 and 109 and the difficulty of controlling peak current in these switches.
Thus, a self-biased non-isolated regulator is needed that is capable of efficiently operating at high switching frequency from the universal AC line voltage and delivering a low output voltage not restricted to the required bootstrap voltage.